Field of the Invention
The present invention relates to the technical field of establishing communication links between surface or land-based equipment and instrumentation arranged in a wellbore. More specifically the invention relates to wireless communication in an annulus of the wellbore, where the annulus may extend into one or more lateral wellbores.
Description of the Related Art
Wireless downhole sensor technology is being deployed in numerous oil and gas wells. In prior art, system components are inductively coupled, which enables remote placement of autonomous apparatus in the wellbore without the need to for any cable connection, cord or battery to neither power nor communicate. These systems make use of a pair of inductive coils where one of the coils usually is casing conveyed, i.e. arranged in the wellbore as part of the casing or liner program, and the other coil is tubing conveyed, which means that it is inserted into the wellbore as part of the completion program. Thus, the pair of coils have to be aligned, usually as part of the completion program, so that they are within a certain distance required for the magnetic field from one coil to be detected by the other coil and vice-versa.
The inductive coils typically consist of a conductor wound around a core. On the sender side a magnetic field will be generated when an electric current is applied to the conductor, while on the receiver side a voltage across the conductor coil will be generated when the magnetic field from the sender attracts the receiver coil. We may say that the receiver coil is harvesting from the sender.
In prior art, power harvesting has been used to provide power to the remote side of the inductive wireless link to power a remote wellbore instrument, so that the instrument has sufficient power to transmit data from the remote wellbore instrument, e.g. sensor data back to the tubing conveyed coil.
The tubing conveyed coil may in turn be connected to a surface control system aboard a platform or ship by a downhole cable, and the control system will eventually receive the information from the remote wellbore instrument so that it can be used to analyze the properties of the wellbore or the surrounding formation.
One problem related to the system of prior art is that the range of the inductive wireless link is limited, and that alignment of the inductive coils is critical for establishment of the link. This may slow down the progress to run and set a completion program for the wellbore due to the inherent need of proximity between the inductive couplers involved.
A further problem is related to the amount of information that can be carried over the inductive wireless link. Information or data is usually in digital form and modulated over the low frequency inductive field that works as a carrier.
U.S. Pat. No. 5,008,664 discloses an apparatus employing a set of inductive coils to transmit AC data and power signals between a downhole apparatus and apparatus of the surface of the earth.
European patent application EP 0678880 A1 discloses an inductive coupling device for coaxially arranged tubular members, where the members an be telescopically arranged and the liner member has a magnetic core assembly constructed from magnetic iron with cylinder sloped ends and the outer member has an annular magnetic assembly aligned with the core assembly.
U.S. Pat. No. 4,806,928 discloses a inner and outer coil assemblies arranged on ferrite cores arranged on a downhole tool with an electrical device and a suspension cable for coupling the electrical device to a surface equipment via the coil assemblies.
Of specific interest for this kind of communication systems, is the possibility for establishing communication with wellbore instruments in lateral wellbores. Lateral wellbores are important for improving production and exploit nearby occurrences of petroleum in the formation.
International patent publication WO2001198632 A1 and US patent application US2011011580 A1 discloses the use of inductive wireless links for establishing communication between a mother wellbore and lateral wellbores. However, in addition to the problems related to prior art above, a new problem related to arrangement of the inductive coils appears. Due to the nature of the lateral junctions, it is difficult to avoid that they become obstacles for the inductive wireless link, so that it becomes hard to establish a reliable communication.